1. Field of the Invention
The invention relates to a start control device of a vehicular power transmission system which performs neutral control (N control) and performs start-time lock-up slip control, and also relates to a start control method. The invention is particularly concerned with control executed when the neutral control is cancelled.
2. Description of the Related Art
In a known type of vehicle, neutral control is performed under which a start clutch is placed in a slipping or released state when the vehicle is stopped so as to bring a power transmission path between an engine and driving wheels into a restricted or interrupted power transmitting condition. Examples of this type of vehicle are described in Japanese Patent Application Publication No. 2005-3193 (JP-A-2005-3193) and Japanese Patent Application Publication No. 2004-353750 (JP-A-2004-353750). More specifically, in JP-A-2005-3193, when certain neutral control execution conditions, such as conditions that the vehicle is stopped with the shift lever placed in the D (running) position, the accelerator pedal is in an OFF (or released) state, and that the vehicle is on a flat road, are satisfied, neutral control for bringing a power transmission path in the automatic transmission into a substantially released or disconnected condition is performed so as to reduce the idling load of the engine and improve the fuel efficiency. Under the neutral control, a clutch (corresponding to a start clutch) for establishing the first-speed gear position of the automatic transmission is placed in a slipping or partially engaged state while the shift lever is kept in the “D” position, so as to substantially interrupt transmission of the engine power to the driving wheels. Then, when the neutral control is cancelled, namely, when the vehicle returns from the neutral control, the clutch is engaged so as to establish a start standby condition in which the vehicle is ready to start immediately.
In another known type of vehicle, start-time lock-up slip control is performed under which a lock-up clutch capable of directly coupling input and output members of a hydraulic power transmission (such as a torque converter or a fluid coupling) is brought into slip engagement when the vehicle is started, as described in JP-A-2005-3193, for example. Generally, under the start-time lock-up slip control, slip control (lock-up slip control, flexible lock-up control) is performed under which the lock-up clutch slips to a given extent upon starting of the vehicle following depression of the accelerator pedal (acceleration ON) to permit a lock-up operation, so as to suppress increase (or reduce the rate of increase) of the engine speed and improve the fuel efficiency, for example.
In the cancellation of the neutral control and the start-time lock-up slip control, both of which are controls for engaging clutches upon starting of the vehicle, the engagement of one of the clutches has an influence on the engagement of the other clutch, and therefore, the respective controls may not be carried out with stability. For example, when the neutral control is cancelled, the start clutch that is in the released state is controlled to be engaged from a point in time at which the brake pedal is released. At this time, in a transient engaging condition of the start clutch (i.e., while the start clutch is in the course of being engaged), the start clutch pressure is controlled based on, for example, the torque transmitted from the engine and the accelerator pedal stroke. If the start-time lock-up slip control is additionally executed during cancellation of the neutral control, the torque transmitted to the start clutch changes since the lock-up clutch is controlled to the apply side under the lock-up slip control. Therefore, the time it takes to complete cancellation of the neutral control may be prolonged if the clutch pressure of the start clutch is controlled to the originally set pressure, and the torque transmitted to the start clutch may be unexpectedly large, resulting in an increase of engagement shock.
When the cancellation of the neutral control is conducted solely (i.e., without involving the lock-up slip control), slipping of the torque converter is utilized so as to suppress (or absorb) shock (e.g., engagement shock or inertia shock) caused by engagement of the start clutch, and variations (fluctuations) in the engine torque that is not stabilized in a transient condition immediately after the accelerator pedal is depressed. In the start-time lock-up slip control, on the other hand, the looseness (slipping, released state) that accommodates a difference in the rotational speed between the input and output members of the hydraulic power transmission that transmits the power of the engine to the automatic transmission is suppressed or reduced due to half-engagement of the lock-up clutch, for example. Accordingly, if the start-time lock-up slip control is additionally executed during cancellation of the neutral control, the shock (torque variation) that occurs upon engagement of the start clutch, variations in the engine torque in the transient condition, etc. are more likely to be transmitted to the output side (namely, is more likely to be transmitted to the user (driver)), which may result in deterioration of the driveability. Therefore, it has been proposed to execute cancellation of neutral control and start-time lock-up slip control one after the other, for example, by starting the start-time lock-up slip control after completion of the neutral control, as described in JP-A-2005-3193.
In this connection, it is desirable, in view of the starting response, to cancel or finish the neutral control as soon as possible while suppressing shock caused by engagement of the start clutch, when the vehicle is started. It is also desirable to switch the lock-up clutch to the apply side at an earliest opportunity so as to improve the fuel efficiency. However, if the cancellation of the neutral control and the start-time lock-up slip control are executed one after the other so as to stably carry out these controls, an effect of improving the fuel efficiency due to the start-time lock-up slip control may be reduced, or the starting response may deteriorate, or shock caused by engagement of the start clutch may be increased. These problems have not been widely known, and it has not been proposed to stably and concurrently carry out control for engaging the start clutch for cancellation of the neutral control and control for engaging the lock-up clutch under the start-time lock-up slip control, so as to cancel the neutral control as soon as possible while suppressing shock, and engage the lock-up clutch as soon as possible for improvement in the fuel efficiency.